duck and chickens

So I have 2 ducks and 2 chicks. They are still pretty young (maybe 3 months old now), and they all nest together in the ad hoc coop I have. I have tried to move the chicks into my chicken coop, where I have an older (1 year) old chicken. She is ok with them, but the chicks wanted out and immediately ran back to the ducks.

I am building a new coop for the ducks. My question is, should I just build it for the chicks also? They have really bonded to the ducks, and right now I don't see splitting them up going very easily. Do I need to or should I just let them hang wih the flock they have developed?

There are two problems that I can see with this situation. If you have a male duck it might try to mate with the chicken. It could kill the hen because their anatomy is different then the chicken. The other problem is the ducks coop will have a lot more mess from the water bowls then what a chicken coop has. I did brood a female duck with two chicks. The duck wanted nothing to do with the other ducks we had and she slept in the chicken coop with them at night.

I'm going to follow this, we have 4 ducks now, in their own house, and want to add 4 silkies to the mix in their own house... but have them share the same run. Not sure if we are positively doing this, or if we're going to separate the run... but I'm always curious about situations that come from folks having both ducks and chickens.